One type of non-contact sensing device is a sensing device that operates by way of capacitance sensing. A capacitance sensing device includes an array of one or more capacitive sensors. The capacitance detected by a capacitive sensing device changes as a function of the proximity of a conductive object or substance to the device. The conductive object or substance can be, for example, a stylus, a user's finger, or detergent in an appliance's detergent reservoir.
In a capacitive sensing device, a change in capacitance detected by a capacitive sensor due to the proximity of a conductive object or substance can be measured by a variety of methods. Regardless of the method, usually an electrical signal representative of the capacitance detected by the capacitive sensor is processed by a processing device, which in turn develops electrical signals representative of the presence of the conductive object or substance in proximity to the capacitive sensing device.
In addition to the presence of a conductive object or substance, a capacitive sensing device is also subject to the effects of environmental factors such as thermal effect due to ambient temperature or moisture effects due to ambient humidity. In a system of identical capacitive sensing devices, changes in ambient temperature will affect the capacitance detected by the sensors because the self-capacitance of the sensors changes with temperature. In general, the capacitance detected by the sensors changes linearly with changes in temperature. In some systems, the capacitance detected has a positive temperature coefficient, and in others will have a negative coefficient. The effect on detected capacitance in all sensors due to changes in ambient temperature is known as temperature drift. With users expecting computing devices that employ capacitive sensors to be able to operate over a wide temperature range, compensation for temperature drift may be required to meet performance specifications.